Shedding by rod photoreceptors after sunrise in fish

Diurnal shedding by retinal rods was studied in wild cutthroat trout, Oncorhyncus clarki, hatcheryrainbow trout, Oncorhyncus mykiss, and the plains killifish, Fundulus zebrinus, by counting the shed tips of rodouter segments ingested as `phagosomes' by pigment epithelial cells. After sunrise, phagosomes increased in allspecies, but fewer occurred in trout, and these were elevated from 3 to 9 hours after sunrise. Shedding occurredearlier in the light period and was more robust in killifish, with phagosomes elevated from 1.5 to 6 hours aftersunset. The data suggest that both production of phagosomes by shedding and their subsequent disposal are slowerat the lower temperatures experienced by trout. Otherwise, rod shedding produced under natural lighting is notappreciably different than that provoked by sudden onset of artificial light.
Key words. Trout: killifish: rod photoreceptor shedding: sunrise.